The convergence of communications and computing technologies is leading to the next generation of networks where many millions of homes and small offices are connected full-time to the Internet, thereby leading to a truly global information age. These developments have sparked an interest in home networking. Several technologies are presently being considered for home networks, one of which is wireless technology. Wireless systems enable non-restrictive communication and eliminate the need to rewire homes for the new networks. Bringing any new technology into the home adds new challenges such as cost, reliability and ease of management, in addition to the optimal use of resources. Home networks need to support a wide variety of devices ranging from a television set to a light bulb.
Random access protocols are well suited for such networks because they can multiplex a wide range of data rates efficiently. Random access protocols are best suited for an ad-hoc wireless network because they require no central control. An ad-hoc wireless network is a collection of wireless nodes that can communicate with each other without the aid of any pre-existing communication infrastructure.
A medium access control (MAC) protocol enables multiple users, via multiple devices, to transmit and receive data on a shared wireless medium in an efficient and orderly manner. Wireless MAC protocols have been extensively researched and a large group of protocols proposed. With the exception of high performance local area network (HIPERLAN), as addressed below, all high speed MAC protocols require a central node such as a base station to coordinate transmissions. Such a central node is not possible in an ad-hoc network. As a result, random access is the multiple access mechanism in the ad-hoc networks. Most wireless random access protocols are designed for low data rates (<2 Mbps) and are based on collision avoidance principles.
Advancements in radio frequency and wireless communication technology are driving the data rates of wireless local area networks (WLAN) higher. In fact, the total bandwidth required by applications like digital video and audio is in excess of 50 Mbps. Unfortunately, recent standards developed for ad-hoc wireless LANs, including HIPERLAN, perform poorly at high data rates and for small packet sizes. The efficiency of random access protocol is determined by how fast collisions within a communication channel are detected and how soon this information can be conveyed to a source node. In wired protocols the ability of a node to listen to a medium while transmitting, and the fact that a collision on the medium is heard by all nodes listening to the medium, results in high performance.
In a wireless medium both the above assumptions do not exist. First, a wireless transceiver cannot be transmitting and listening to the medium at the same time due to self-interference. Basically, any transmitted signal that leaks into the receiver usually has a much higher energy than the received signal, and hence, transceivers cannot listen and transmit at the same time. In the wireless medium the signal strength falls off as a function of distance. Hence, depending on the position of a node relative to the source node, channel sensing will produce different results. Consider, for example, the scenario illustrated in FIG. 1 where node 106 is in radio coverage of nodes 104 and 108 and node 108 is in range of nodes 106 and 112. If node 106 is transmitting, nodes 104 and 108 sense the communication channel is busy while node 112 thinks the channel is idle. A transmission from node 112 will corrupt data reception at node 108. Unlike in a wired media, two simultaneous transmissions do not imply a collision. A collision occurs when the destination node cannot decode a transmission. Therefore, the destination node is the only node that can identify a collision. When a collision is detected, this information should be conveyed to the sending node, so that it can abort its transmission and minimize wasted channel capacity. The main reason for the low efficiency of these protocols is the lack of an efficient mechanism for the destination node to tell the sending node about the current transmission.
Those skilled in the art are perplexed by the problem of determining how to enable the destination node to convey information about its state of transmission, either idle or collision, to the source node in a timely manner. Unfortunately, a feedback channel is not available in current wireless systems since transmitted and received signal strengths differ by orders of magnitude and current technology does not provide enough isolation between transmit and receive paths to allow correct demodulation of the received signal when transmitting data.
Current collision avoidance protocols try to minimize collisions by exchanging handshaking messages to reserve the communication channel for data transmission. This handshaking can be considered as duplexing feedback information in time. Handshaking requires a node to switch between transmit and receive modes. More importantly, intrinsic to the handshaking protocols are turn-around periods called mute-deaf times. These are periods, when the transceiver is switching from transmit mode to receive mode or vice-versa. During these periods the transceiver can neither listen nor transmit. These switching time constraints are a large overhead at high data-rates.
Another alternative is to use a different frequency bands for the data channel and the feedback channel. This is referred to as frequency duplexed feedback, which is used by busy-tone protocols. However, this has a higher hardware cost, and low spectral efficiency, although it results in very efficient rate-scalable wireless MAC protocols. The high cost results from the requirement of two transceivers, one for the data channel and the other for the feedback channel. Even though the feedback channel is a narrow-band channel, a significant amount of bandwidth needs to be allocated for it because it is hard to implement band-pass filters that have small bandwidth and sharp cutoff.
One example of such a protocol is the receiver initiated busy tone multiple access protocol (RI-BTMA). Operation of the RI-BTMA protocol is relatively simple. When a node has data to transmit, it senses the busy-tone channel for any ongoing transmissions. If the channel is idle, the node will initiate a data transmission. A node will then send a busy tone in an out of band channel after it knows that it is the destination for this packet. If the node receives a busy tone signal it will continue with the transmission. Else, it would abort the transmission and retry after a random time. The efficiency of this protocol is limited by how fast the destination node can be determined and the busy tone asserted. As a result, such protocols are efficient over a wide range of data rates unlike handshaking protocols. However, the hardware cost of two transceivers and the inefficiency of a separate busy tone channel, which is attributed to the significant amount of bandwidth that has to be allocated to convey one bit of information, prohibits the use of such protocols.